In order to break the limit of electrical wires in LSIs (large scale integrated circuits) in terms of the high speed operation, signal communication between chips at a higher speed has been attempted by forming an optical circuit on an Si substrate. An optical circuit consists of optical elements such as an optical modulator that modulates an input signal, an optical receiver that receives a light signal, and optical function elements such as a splitter that splits light.
Light is inputted to these optical function elements via an optical waveguide that propagates a light signal. In addition, light enters the optical circuit from the outside, or light enters the optical circuit from a light source formed on the Si substrate.
In order to make the miniaturization of optical circuit elements easier, it is better to form a light source on an Si substrate than allow light to enter through an optical fiber. In order to realize a light source on an Si substrate, technologies such as direct growth, flip-chip bonding and wafer bonding are available. Fabrication using a technology for bonding substrates together is characterized by such advantages as mass production and the integration of multiple function elements, and thus, is developing.
For example, it has been proposed to bond an active layer made of a III-V compound semiconductor onto an Si substrate using SiOx (see Non-Patent Document 1). In this proposal, the active region made of the III-V compound semiconductor and the Si waveguide are provided as tapered waveguides so that the spot size is widened in order to allow light to transit efficiently from the active region made of the III-V compound semiconductor to the Si waveguide.
It has also been proposed to bond an active layer made of a III-V compound semiconductor onto an Si substrate through wafer bonding (see Non-Patent Document 2). In this proposal, the transition from the active region made of the III-V compound semiconductor to the Si waveguide takes place by providing a tapered waveguide on the active region side made of the III-V compound semiconductor to widen the spot size. In addition, a reflection structure is provided on the Si waveguide side.
Meanwhile, it has been proposed to bond a quantum dot laser having excellent high temperature properties as a light source (see Non-Patent Document 3). In this proposal, a quantum dot laser is bonded through wafer bonding, and its operation as a light source on an Si substrate is confirmed.